SEGREGATION. 475 



are alike, and unlike motions in units that are unlike. If 

 then, in an aggregate containing two or more orders of 

 mixed units, those of the same order will be moved in the 

 same way, and in a way that differs from that in which units 

 of other orders are moved, the respective orders must segre- 

 gate. A group of like things on which are impressed mo- 

 tions that are alike in amount and direction, must be trans- 

 ferred as a group to another place, and if they are mingled 

 with some group of other things, on which the motions im- 

 pressed are like each other, but unlike those of the first 

 group in amount or direction or both, these other things must 

 be transferred as a group to some other place the mixed 

 units must undergo a simultaneous selection and separation-. 

 In further elucidation of this process, it will be well here 

 to set down a few instances in which we may see that, other 

 things equal, the definiteness of the separation is in propor- 

 tion to the definiteness of the difference between the units. 

 Take a handful of any pounded substance, containing frag- 

 ments of all sizes; and let it fall to the ground while a 

 gentle breeze is blowing. The large fragments will be 

 collected together on the ground almost immediately under 

 the hand; somewhat smaller fragments will be carried a 

 little to the leeward; still smaller ones a little further; and 

 those minute particles which we call dust, will be drifted a 

 long way before they reach the earth : that is, the integration 

 is indefinite where the difference among the fragments is 

 indefinite, though the divergence is greatest where the 

 difference is greatest. If, again, the handful be made up of 

 quite distinct orders of units as pebbles, coarse sand, and 

 dust these will, under like conditions, be segregated with 

 comparative definiteness : the pebbles will drop almost verti- 

 cally ; the sand will fall in an inclined direction, and deposit 

 itself within a tolerably circumscribed space beyond the 

 pebbles; while the dust will be blown almost horizontally to 

 a great distance. A case in which another kind of force 

 comes into play, will still better illustrate this truth. 



